Much controversy exists over the significance of the knee laxity exam. This has resulted, in part, from a lack of knowledge as to the restraining functions of the various ligaments and capsular structures. During the first grant period we measured the ligament restraints to the so-called "straight instabilites." These are anterior-posterior drawer and straight medial-lateral opening. In the proposed renewal period we will study the restraining action of the ligaments to the rotatory instabilities. These instabilities involve combined rotational and translational knee motion. The rotatory motion is that associated with axial tibial rotation. The translations are anterior-posterior drawer and medial-lateral shifting of the tibia. We propose to address the problem of defining the significance of rotatory laxities, with regard to injury, by applying three distinct methodologies. These are described below in separate projects. They include: 1) Measuring the three-dimensional restraining forces of the ligaments to well defined and reproducible rotations of the tibia, 2) measuring the three-dimensional motions which occur during the various clinical exams for rotatory instability along with the changes in motion associated with specific ligament injuries, and 3) synthesizing these results on ligament function into a mathematical model capable of testing our understanding of both the effects of the various individual factors (i.e., ligaments, joint geometry, compressive forces, etc.) and their complex interactions.